A heat-sensitive recording method has many advantages in that no particular developing step is required, (2) if paper is used as a support, the recording material can have a quality akin to that of plain paper, (3) handling of the recording material used is easy, (4) the images recorded have high color density, (5) this method can be effected using a simple and cheap apparatus and (6) no noise is caused during recording. Therefore, heat-sensitive recording materials have recently enjoyed a markedly increasing demand, particularly for use with a facsimile or printer, and have come to be used for many purpose.
From this situation, it has been desired to devise transparent heat-sensitive recording materials which enable direct recording with a thermal head in order to adapt them for multicolor development, or to make them usable for an overhead projector (hereafter abbreviated as OHP).
However, all heat-sensitive layers of known heat-sensitive recording materials to which the recording with a thermal head is applicable are opaque, and desired transparency cannot be realized even if a transparent support is employed as the support on which such heat-sensitive layers are to be provided.
On the other hand, conventional dichromatic recording methods which are applicable to heat-sensitive recording materials are classified into two types, a color-mixing type and a decolorizing type.
However, developed color image in the lower layer of the above-described decolorizing type dichromatic heat-sensitive recording material suffers from the defect that it has not only hue affected adversely by opacity of the upper heat-sensitive layer but also insufficient sharpness in itself.
As the result of our study on removal of the foregoing defect of conventional heat-sensitive materials, it has now been found that a heat-sensitive layer excellent in transparency can be obtained by selecting as a color former a combination of an organic base and a colorless or light colored color-forming substance capable of developing its color by reaction with said organic base, microcapsulating the latter and dispersing the former in the form of emulsion under a definite condition to prepare a coating composition, thus achieving the present invention.